Heterogeneous Networks
In a typical cellular radio system, mobile terminals (also referred to as user equipment, UEs, wireless terminals, terminal devices, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks, which provide access to data networks, such as the Internet, and/or to the public-switched telecommunications network (PSTN). A RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station (also referred to as a base station, a RAN node, a “NodeB”, and/or an enhanced NodeB or “eNB”). A cell area is a geographical area over which radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband data services based on, for example, WCDMA (Wideband Code-Division Multiple Access), HSPA (High-Speed Packet Access), and Long Term Evolution (LTE) wireless technologies. Fueled by the introduction of new devices designed for data applications, end user performance requirements continue to increase. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by high-speed wireless data networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently are desired.
Techniques to improve downlink performance may include Multiple-Input-Multiple-Output (MIMO) multi-antenna transmission techniques, multi-flow communication, multi-carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for wireless networks may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. Currently, so-called heterogeneous networks are being developed by members of the 3rd-Generation Partnership Project (3GPP), as discussed, for example, in: RP-121436, “Study on UMTS Heterogeneous Networks,” TSG RAN Meeting #57, Chicago, USA, 4-7 Sep. 2012; R1-124512, “Initial considerations on Heterogeneous Networks for UMTS,” Ericsson, ST-Ericsson, 3GPP TSG RAN WG1 Meeting #70bis, San Diego, Calif., USA, 8-12 Oct. 2012; and R1-124513, “Heterogeneous Network Deployment Scenarios,” Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1#70bis, San Diego, Calif., USA, 8-12 Oct. 2012.
A homogeneous network is a network of base stations (also referred to as NodeBs, enhanced NodeBs, or eNBs) in a planned layout, providing communications services for a collection of user terminals (also referred to as user equipment nodes, UEs, terminal devices, and/or wireless terminals), in which all base stations typically have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may generally offer unrestricted access to user terminals in the network, and each base station may serve roughly a same number of user terminals. Current cellular wireless communications systems in this category may include, for example, GSM (Global System for Mobile communication), WCDMA, HSDPA (High Speed Downlink Packet Access), LTE (Long Term Evolution), WiMAX (Worldwide Interoperability for Microwave Access), etc.
In a heterogeneous network, low power base stations (also referred to as low power nodes (LPNs), micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit nodes, RRU nodes, small cells, RRUs, etc.) may be deployed along with or as an overlay to planned and/or regularly placed macro base stations. A macro base station (MBS) may thus provide service over a relatively large macro cell area, and each LPN may provide service for a respective relatively small LPN cell area within the relatively large macro cell area.
Power transmitted by an LPN may be relatively small, e.g., 2 Watts, compared to power transmitted by a macro base station, which may be 40 Watts for a typical macro base station. An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in the coverage provided by the macro base stations, and/or to off-load traffic from macro base stations, such as to increase capacity in a high traffic location or so-called hot-spot. Due to its lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition.
Thus, a heterogeneous network features a multi-layered deployment of high-power nodes (HPNs), such as macro base stations, and low-power nodes (LPNs), such as so-called pico-base stations or pico-nodes. The LPNs and HPNs in a given region of a heterogeneous network may operate on the same frequency, in which case the deployment may be referred to as a co-channel heterogeneous deployment, or on different frequencies, in which case the deployment may be referred to as an inter-frequency or multi-carrier or multi-frequency heterogeneous deployment.
Inter-Cell Interference Coordination
Inter-cell interference presents a big performance issue for cell edge users. In a heterogeneous network, the impact of inter-cell interference can be worse than is generally seen in homogeneous networks, due to large differences between the transmit power levels of macro base stations and LPNs. This is illustrated in FIG. 1, which illustrates a heterogeneous network deployment 100 in which two pico-nodes 130 have coverage areas that fall within the coverage area 120 of macro node 110. The cross-hatched regions 140 in FIG. 1 cover a region between an outer circle and an inner circle around each LPN. The inner circle represents an area where the received power from the LPN is higher than that from the macro base station. The outer circle represents an area where the path loss to the LPN base station is smaller than that to the macro base station.
The cross-hatched area 140 between the inner and outer circles is often referred to as the “imbalance zone.” This imbalance zone 140 could potentially be an LPN range-expansion area because, from the uplink (terminal-to-base-station) perspective, the system would prefer that the terminal still be served by the LPN within this area. However, from the downlink (base-station-to-terminal) perspective, terminals at the outer edge of such an imbalance zone, such as terminal 150a in FIG. 1, experience a very large received-power difference between the macro and LPN layers. For example, if the transmit power levels are 40 watts and 1 watt for the macro node and LPN, respectively, this power difference can be as high as 16 dB. In contrast, terminals relatively far away from the pico-nodes 130, such as mobile terminal 150b, are not affected, because the received powers from the LPNs are significantly less than that received from the macro base station 110.
As a result of these power differences, if a terminal in the range-expansion zone is served by a LPN cell and the macro cell is serving another terminal at the same time, using the same radio resources, then the terminal served by the LPN is subject to very severe interference from the macro base station.
Inter-cell interference coordination (ICIC) is supported in LTE networks, and is managed by signaling sent between eNodeBs via the eNodeB-to-eNodeB X2 interface. Each cell can signal to its neighboring cells, identifying high-power resource blocks in the frequency or time domains. This allows the neighboring cells to schedule cell-edge users in such a way as to avoid these high-power resource blocks. Such a mechanism can be used to reduce the impact of inter-cell interference.
Small-Cell on/Off
One of the mechanisms under development by members of the 3rd-Generation Partnership Project (3GPP) for interference avoidance and coordination among small cells is a small-cell on/off feature. According to this feature, a small cell may be turned on and off from time to time, where the “on” and “off” periods may depend on the criteria or application.
The small-cell on/off feature may be implemented in semi-static or dynamic versions. With semi-static small-cell on/off, in which the on/off periods are very long, compared to the system's transmission-time intervals, criteria for cell on/off can be traffic load, terminal device arrival/departure, etc. On the other hand, with dynamic small-cell on/off, the small cell can be turned on and off at the level of a single subframe. The criteria in this case can be packet arrival/completion or interference coordination and avoidance (e.g., to reduce interference towards other nodes or UEs). This means that the cell turns off at the subframe boundary (or end of current subframe) when the transmission of packet is completed and turns on at the next subframe boundary where a packet arrives.
In addition to its advantages in reducing interference, the small-cell on/off feature can also provide energy savings. Some preliminary evaluation of the energy saving impact of the small-cell on/off is presented in the 3GPP document, “Small cell enhancements for E-UTRA and E-UTRAN; Physical layer aspects,” 3GPP TR 36.872, ver. 12.0.0 (available at www.3gpp.org).
There are three primary operational modes of the small-cell on/off feature:                Handover: In this mode a terminal device in CONNECTED mode is always attached to a cell. Due to increased traffic demand, for example, the network may decide to offload all or part of traffic for a given terminal device by handover to a small cell. The small cell, which may be “off”, wakes up to serve the terminal device. The handover time in this case depends on the backhaul delay and the handover execution time. After completion of the transmission and/or reception of data the terminal device goes to IDLE mode or handed over to another cell, and the small cell can be turned off.        SCell only: In this mode a terminal device supporting carrier-aggregation (CA) is connected to a primary carrier or primary cell (PCell), and the network configures a secondary carrier or secondary cell (SCellI) that can be turned on or off. If the network decides to offload the terminal device traffic to the SCell, then the SCell is turned on.        Serving cell (which may be the PCell in a CA scenario): In this mode a cell can be either on or off when a terminal device is connected to it. The procedures for radio resource management (RRM), radio link management (RLM) and channel-state information (CSI) measurements must be designed for this case.        
Discovery Signal
In small-cell on/off deployments where the eNB can be off for long periods of time, a discovery signal might be needed to assist the terminal device with the measurements that it must perform. (These measurements are discussed in further detail below.) The discovery signal needs to support the properties for enabling RRM measurements, RLM-related procedures, and coarse time/frequency synchronization. To make the terminal device measurements possible, the eNB must wake up periodically (e.g., once every 80 or 160 milliseconds) and transmit the discovery signal so that it can be used by the terminal device for mobility related operations such as cell identification, RLM and measurement.
Since the discovery signal is generally rather sparse in time, it is desirable that the terminal device is able to make a meaningful measurement in one instance of the discovery signal, rather than having to wait for multiple instances that may occur tens or hundreds of milliseconds apart. In addition, to make measurements based on fewer samples in time more reliable, a discovery signal that only includes a few samples per instance may need to be sent over a wide bandwidth (e.g., the whole bandwidth used by the eNB or by the system).
Considering the above desired properties, one option for such discovery signals is to use currently existing signals as the discovery signal, such as the existing primary/second synchronization signals (PSS/SSS), common reference symbols (CRS), channel-state information reference symbols (CSI-RS), and/or positioning reference symbols (PRS). This enables UEs to reuse current functionality to a large extent, and also has the potential of creating the least impact to the system design. Another option is to use currently existing signals that are augmented in some manner. A third option is to design a completely new discovery signal.
Another alternative is to base the discovery signal on uplink (UL) signals. A UL discovery signal can be an existing signal such as sounding reference signals (SRS), etc., or a new signal. Desirable properties of an UL discovery signal are similar to those for a DL discovery signal. However, since the mechanism of the discovery in the UL can be different, the design of such signal can be different too. The UL discovery signal can be used for uplink measurements (e.g., for UL transmit-timing accuracy) or for measurements that use both UL and DL discovery signals, such as UE Rx-Tx time difference, eNB Rx-Tx time difference measurements, etc.
Terminal Device Measurements
To support different functions such as mobility, which in turn includes the functions of cell selection, cell reselection, handover, RRC re-establishment, connection release with redirection, etc., as well as to support other functions such as minimization of drive tests, self-organizing network (SON), positioning, etc., the terminal device is required to perform one or more radio measurements (e.g., timing measurements, signal strength measurements or other signal quality measurements) on signals transmitted by neighboring cells, i.e., by cells other than the cell serving the terminal device. Prior to performing such measurements the terminal device generally has to identify the cell from which a signal is sent, and determine the cell's physical cell identity (PCI). Therefore, PCI determination can also be considered a type of a measurement.
The terminal device receives measurement configuration or assistance data/information, which is a message or an information element (IE) sent by the network node (e.g., a serving eNodeB, positioning node, etc.) to configure the terminal device to perform the requested measurements. For example, the measurement configuration may contain information related to the carrier frequency to be measured, a radio-access technology (RAT) or RATs to be measured, a type of measurement (e.g., Reference Signal Received Power, or RSRP), whether higher-layer time-domain filtering should be performed, measurement bandwidth related parameters, etc.
The measurements are done by the terminal device on the serving cell as well as on neighbor cells, over some known reference symbols or pilot sequences. The measurements are done on cells on an intra-frequency carrier, inter-frequency carrier(s) as well as on inter-RAT carriers(s) (depending upon the UE's capability for supporting a particular RAT or RATs).
In RRC connected state, the terminal device can perform intra-frequency measurements without using measurement gaps (i.e., intervals in which the mobile terminal receiver may re-tune to another frequency and/or configure itself for a different RAT). However, as a general rule the terminal device performs inter-frequency and inter-RAT measurements in measurement gaps unless it is capable of performing them without gaps. To enable inter-frequency and inter-RAT measurements that require gaps, the network has to configure measurement gaps for the terminal device. Two periodic measurement gap patterns, both with a measurement gap length of 6 milliseconds, are defined for LTE:                Measurement gap pattern #0 with repetition period 40 milliseconds; and        Measurement gap pattern #1 with repetition period 80 milliseconds.In High-Speed Packet Access (HSPA) networks, the inter-frequency and inter-RAT measurements are performed in compressed mode gaps, which are also a type of network-configured measurement gap.        
Some measurements may also require a terminal device to measure the signals transmitted by the terminal device in the uplink. The measurements are done by the terminal device in RRC connected state or in CELL_DCH state (in HSPA) as well as in low activity RRC states (e.g., idle state, CELL_FACH state in HSPA, URA_PCH and CELL_PCH states in HSPA, etc.). In a multi-carrier or carrier aggregation (CA) scenario, the terminal device may perform the measurements on the cells on the primary component carrier (PCC) as well as on the cells on one or more secondary component carriers (SCCs).
These measurements are done for various purposes. Some example measurement purposes are: mobility, positioning, self-organizing network (SON), minimization of drive tests (MDT), operation and maintenance (O&M), network planning and optimization, etc. The measurements are typically performed over time durations on the order of a few hundreds of milliseconds to a few seconds. The same measurements are generally applicable to both single-carrier and carrier aggregation scenarios. However in carrier aggregation scenarios the specific measurement requirements may be different. For example, the measurement period may be different in carrier aggregation scenarios; i.e., it can be either relaxed or more stringent depending upon whether a secondary component carrier (SCC) is activated or not. This may also depend upon the UE's capability, i.e. whether a carrier aggregation-capable terminal device is able to perform measurements on an SCC with or without gaps.
Examples of mobility measurements in LTE include:                Reference symbol received power (RSRP); and        Reference symbol received quality (RSRQ).        
Examples of mobility measurements in HSPA are:                Common pilot channel received signal code power (CPICH RSCP); and        CPICH Ec/No.        
An example of mobility measurements in GSM/GERAN is:                GSM carrier RSSI.        
Examples of mobility measurements in CDMA2000 systems are:                Pilot strength for CDMA2000 1×RTT; and        Pilot strength for HRPD.        
Mobility measurements may also include the step of identifying or detecting a cell, which may belong to LTE, HSPA, CDMA2000, GSM, etc. Cell detection comprises identifying at least the physical cell identity (PCI) and subsequently performing the signal measurement (e.g., RSRP) of the identified cell. The terminal device may also have to acquire the cell global ID (CGI) of a terminal device. In HSPA and LTE the serving cell can request the terminal device to acquire the system information (SI) of the target cell. More specifically, the SI is read by the terminal device to acquire the cell global identifier (CGI), which uniquely identifies a cell of the target cell. The terminal device may also be requested to acquire other information such as CSG indicator, CSG proximity detection, etc., from the target cell.
Examples of positioning measurements in LTE are:                Reference signal time difference (RSTD); and        UE RX-TX time difference measurement.        
The UE RX-TX time difference measurement requires the terminal device to perform measurement on the downlink reference signal as well as on the uplink transmitted signals.
Examples of other measurements which may be used for radio link maintenance, MDT, SON or for other purposes are:                Control channel failure rate or quality estimate e.g.,                    Paging channel failure rate, and            Broadcast channel failure rate;                        Physical layer problem detection e.g.,                    Out of synchronization (out of sync) detection,            In synchronization (in-sync) detection,            Radio link monitoring, and            Radio link failure determination or monitoring.                        
Still other measurements performed by the terminal device include channel-state-information (CSI) measurements, which are used for scheduling, link adaptation, etc. by the network. Examples of CSI measurements are CQI, PMI, RI, etc.
The terminal device also performs measurements on the serving cell (also referred to as the primary cell, or PCell) in order to monitor the serving cell performance. These are called radio link monitoring (RLM) or RLM-related measurements in LTE. For RLM the terminal device monitors the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the serving or PCell.
In order to detect out-of-sync and in-sync status for a given radio link, the terminal device compares an estimated quality of the radio link with the thresholds Qout and Qin, respectively. The thresholds Qout and Qin are defined to correspond to signal quality levels below which the downlink radio link cannot be reliably received, and respectively correspond to 10% and 2% block-error-rates for a hypothetical PDCCH transmissions.
Radio measurements performed by the terminal device are used by the terminal device for one or more radio operational tasks. Examples of such tasks are reporting the measurements to the network, which in turn may use them for various tasks. For example, in RRC connected state the terminal device reports radio measurements to the serving node. In response to the reported terminal device measurements, the serving network node takes certain decisions, e.g., it may send a mobility command to the terminal device for the purpose of cell change. Examples of cell change are handover, RRC connection re-establishment, RRC connection release with redirection, primary cell (PCell) change in CA, Primary Component Carrier (PCC) change in PCC, etc. An example of cell change in idle or low activity state is cell reselection. In another example, the terminal device may itself use the radio measurements for performing tasks e.g., cell selection, cell reselection, etc.
A radio network node (e.g., base station) may also perform signal measurements. Examples of radio network node measurements in LTE are propagation delay between terminal device and itself, UL SINR, UL SNR, UL signal strength, Received Interference Power (RIP), etc. An eNB or other radio network node may also perform positioning measurements, which are described in a later section.
A typical serving cell or neighbor cell measurement quantity is based on the non-coherent averaging of 2 or more basic non-coherently averaged samples, each of which may be the result of the non-coherent averaging of one or more short (e.g., 1-millisecond) coherent measurements. The exact sampling for any given measurement depends upon the implementation of the terminal device or network node radio, and is generally not specified.
An example of RSRP measurement averaging in E-UTRAN is shown in FIG. 2. The figure illustrates that the terminal device obtains the overall measurement quantity result by collecting four non-coherent averaged samples or snapshots (each of 3-milliseconds length, in this example) during a physical layer measurement period (e.g., 200 milliseconds), when no discontinuous receive (DRX) is used or when DRX cycle is not larger than 40 milliseconds. Every coherent averaged sample is 1-millisecond long. The sampling also depends upon the length of the DRX cycle. For example, for DRX cycles greater than 40 milliseconds, the terminal device typically takes one sample every DRX cycle over the measurement period. A similar measurement sampling mechanism is used for other signal measurements by the terminal device and also by the base station for UL measurements.